Titanium dioxide nucleating agent systems for foamable polymer compositions

ABSTRACT

The invention concerns nucleating agent systems for foamable polymer compositions, preferably fluoropolymer resins, having foam cell nucleation sites using titanium dioxide based nucleating agents, and foamable polymers comprising the nucleating agents. The nucleating agent systems comprise TiO 2 , certain inorganic salts, and, optionally, sulfonic acid salts and/or phosphonic acid salts. The invention also concerns foamable composition concentrates and foamed thermoplastic insulation material comprising fluoropolymer and the nucleating agent systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a divisional of Ser. No. 09/964,180 filed Sep. 25,2001, currently pending, which claims the benefit of U.S. ProvisionalApplication No. 60/236,991 filed Sep. 29, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates nucleating agent systems for foamable polymercompositions, preferably fluoropolymer resins, having foam cellnucleation sites using titanium dioxide (“TiO₂”) based nucleatingagents. The nucleating agent systems comprise synergistic amounts TiO₂,certain inorganic salts, and optionally, sulfonic acid salts and/orphosphonic acid salts. The fluoropolymers useful for making the foamablecompositions described herein are organic polymeric materials comprisingat least about 35 weight percent fluorine and having a melt flow rate(“MFR”) of about 1 g/10 min. to about 100 g/10 min. as measuredaccording to ASTM Standard D1238. The nucleating agents comprise TiO₂ inamounts from about 50 parts per million (“ppm”) to about 2,000 ppm byweight based on the total weight of the composition, inorganic salt inamounts from about 25 ppm to about 3,000 ppm by weight based on thetotal weight of the composition and, optionally, sulfonic acid saltsand/or phosphonic acid salts in amounts of from 0 to about 3,000 ppm byweight based on the total weight of the composition. The foamedfluoropolymer compositions comprising the nucleating agents addressedherein have good structure and cell size, smaller than can be achievedwith other systems, and have exemplary performance characteristicsparticularly for thin walled foamed insulation for conductive materials.

2. The Related Art

Foamed polymer compositions generally comprise nucleating agents to forma cell structure during processing. Conventional nucleating agents areboron nitride (“BN”), calcium carbonate, magnesium dioxide, lead oxide,barium oxide, antimony oxide, magnesium carbonate, zinc carbonate,barium carbonate, carbon black, graphite, alumina, calcium silicate,calcium metasilicate and calcium sulfate. Polytetrafluoroethylene(“PTFE”) is described in U.S. Pat. No. 4,304,713 as a nucleating agentfor making dielectric compositions for use in coaxial cables, and U.S.Pat. No. 5,314,925 discusses fluoropolyolefin nucleating agents formolded thermoplastics. U.S. Pat. No. 5,716,665 addresses the use ofPTFE, as well as BN, silicon nitride, alumina, talc and zinc sulfide, asnucleating agents for foamable solid compositions based onthermoprocessible perfluoropolymers.

Despite the number of nucleating agents disclosed in the art, BN hastraditionally been the most preferred, particularly for making foamedinsulation for conductive wire or other materials. BN, however, mustgenerally be used in amounts from 0.5% to 2.0% by weight of totalcomposition (5,000 ppm to 20,000 ppm) for effective nucleation offluoropolymers. BN is a rather expensive, ranging in price from about$60 per pound to about $90 per pound. The price of BN is a significantdrawback to the use of BN in all applications, and, accordingly, the artof nucleating agents for foamed polymer compositions is evolving toobtain nucleating agent systems which eliminate or reduce the quantityof BN while maintaining or improving on the performance characteristicsneeded for many applications.

Conductive wire is often used to transmit electronic signals. Theconductor or conductive material must be protected, or insulated, andthermoplastic coatings in a molten state foamed and formed onto andaround the conductor or conductive material are used for this purpose.The thermoplastic materials are selected on the basis of severalcriteria including high dielectric strength, low dielectric constant,and low dissipation factor. It has previously been found that if thethermoplastic material is foamed as it is applied to wire, thedielectric constant is desirably reduced due to the formation ofnumerous small non-interconnected cells in the foam. U.S. Pat. No.3,072,583 discloses a nucleated foaming process for extrudingperfluorinated polymer foam around transmission wire with a gas blowingagent. Foamed insulation using other thermoplastic materials, especiallypolyolefins, is also known in the art.

In general, after a conductor or conductive material is insulated with afoamed polymer, it is twisted into pairs (twinning), then four pairs aretypically twisted together, and a polymer jacket is added to make thefinal cable. Adequate crush force of the insulation material isnecessary so that the insulation material will maintain its structuralintegrity, e.g. foamed cell structure, during twinning and consumer use.Structural integrity can be achieved by minimizing or eliminating theamount of elongated cells in the foamed polymer structure and maximizingthe amount of smaller sized cells. Elongated cells are ellipticallyshaped and are merely voids filled with gas, such as nitrogen, andprovide poor mechanical strength to the foamed polymer. As such, duringtwinning, insulation material with a significant number of elongatedcells has a tendency to crush which results in a final cable with poorelectrical properties. Nucleating agents must be selected to obtainspherical shaped cells which provide the mechanical strength fordurability, particularly during twinning operations, to obtain a finalcable with good mechanical strength and good electrical properties.Thus, foamed polymer compositions require effective nucleating agentsystems in effective amounts to make a foamed polymer with theappropriate structural integrity and performance properties,particularly for use as insulation for conductive materials.

In general, reduction in foam cell size is desired for thermoplasticresins. In particular, for thin wall foams (e.g. 5-10 mils) small voidsto maximize the number of bubbles are desired for mechanical andelectrical properties. This is especially so for smaller wireconstructions of interest in the electronics field, so that, forexample, foam cell dimensions will be small with respect to the radialdimension of the thin insulation.

Void space of foamed polymers is also important with regard to thecapacitance of the foamed insulation material. It is important that theconductor with foamed insulation (primary) have a capacitance with lowstandard deviation. If the voids in the foamed insulation are notuniform, then capacitance variation can arise along a coated wire.Accordingly, small uniform bubbled distribution within the insulationmaterial, which arises by creation of spherical shaped cells, willresult in uniform capacitance. Thus, the nucleating agents are selectedto obtain the uniform cell distribution within the foamed insulationmaterial for stable capacitance.

Foamed fluoropolymers are also useful in applications other than wireinsulation. Examples of foam structures include foamed sheet forelectrical insulation, thermal insulation and cushioning, foamed pipeand tubing and injection molded pieces. Structural integrity and theminimization or elimination of elongated cells are also important forthese applications.

A 0.5 to 2.0 weight percent loading of BN alone (5,000 ppm to 20,000ppm) is typically used to provide foam cell nucleation in fluoropolymerresins having uniform, small cell structure. U.S. Pat. No. 4,764,538discusses the use of synergistic combinations of BN in amounts of 0.02to 2.0% by weight, and certain inorganic salts in amounts of 25 ppm to3,000 ppm. The preferred range for BN in this system is 500 ppm to10,000 ppm, and the minimum BN concentration exemplified in the patentis 2,500 ppm.

U.S. Pat. No. 4,877,815 describes a class of sulfonic and phosphonicacids, and salts of the acids, which are said to be effective nucleatingagents for a wide variety of thermoplastic materials. The patentdiscusses the combination of certain sulfonic and phosphonic acids withBN, alone or combined with calcium tetraborate for foamingtetrafluoroethylene/hexafluoropropylene copolymers. U.S. Pat. No.5,610,203 concerns the combination of BN crystals grown to final size,inorganic salt and/or sulfonic or phosphonic acids as nucleating agents.

TiO₂ is a white inorganic material and has a current market price ofabout $1 to about $2 per pound. TiO₂ is described in the art as anucleating agent for foamed polymers, however, through the inventiondescribed herein it is discovered that TiO₂ used in certain quantitiesin conjunction with certain inorganic salts and, optionally, sulfonicacid salts and/or phosphonic acid salts act synergistically to result infoamed fluoropolymer compositions with good cell structure and size thancan be achieved with known systems, including conventional nucleatingsystems comprising TiO₂. TiO₂ is also known to have use as a pigment andin topical medicinal preparations.

U.S. Pat. No. 4,615,850 discusses foamable polyvinylidene fluoridepolymers using a nucleating agent having an average particle size ofless than 2 microns. A myriad of nucleating agents are referenced in thepatent, including TiO₂, however the patent provides no disclosure ofTiO₂ used in conjunction with inorganic salts, sulfonic acid salts orphosphonic acid salts, and only foamed polymer compositions comprisingcalcium carbonate nucleating agents are exemplified. U.S. Pat. No.5,696,176 addresses foamable polyester compositions comprising anucleating agent of a polyolefin and inorganic materials, such as TiO₂,talc, chopped fiberglass, alumina, clay and fumed silica. U.S. Pat. No.5,710,186 discusses closed cell polymer foam comprising TiO₂, surfacetreated with either organic materials or organic and inorganic materialswhich particulate is used to prevent or reduce agglomeration and reducethermal conductivity. Nucleating agents in coating compositionscomprising glass microballoons and pigments such as TiO₂ and ZnO arediscussed in U.S. Pat. No. 4,594,368.

An object of the invention is nucleating agents, and nucleating agentsystems, which enhance the effective nucleation of polymers,particularly fluoropolymers, and provide exemplary cell structure, whichdo not comprise BN and, thus, have lesser cost than nucleating agentsystems comprising BN while having enhanced nucleating effectiveness.

It is a further object of the invention to obtain foamed polymercompositions comprising these nucleating agents for use in conductivewire jacketing and insulation, or otherwise.

These and other objectives are achieved by the nucleating agentsdescribed herein comprising a synergistic combination of TiO₂, inorganicsalts and, optionally, sulfonic acid salts and/or phosphonic acid salts.It was unexpectedly discovered that TiO₂ in the TiO₂ based nucleatingagent systems described herein could be used in concentrationssignificantly less than the BN concentrations in conventional systems,and that nucleating agent systems comprising TiO₂, certain inorganicsalts and, optionally, sulfonic acid salts and/or phosphonic acid saltshad better efficiency and provided smaller cell size at very lowconcentrations than nucleating systems in the art. Thus, a substantialcost saving can be achieved with the nucleating agent systems describedherein comprising TiO₂ and the foamed polymers comprising thesenucleating agent systems.

In the present Specification, all parts and percentages are byweight/weight unless otherwise specified.

SUMMARY OF THE INVENTION

The invention pertains to nucleating agent systems comprisingsynergistic amounts of TiO₂, certain inorganic salts and, optionally,sulfonic acid salts and/or phosphonic acid salts. The composition of thenucleating agent system will depend on the specific thermoplastic resin,combination of components and the desired foam structure. Typically,however, the nucleating agents comprise TiO₂ in amounts from about 50ppm to about 2,000 ppm, preferably about 100 ppm to about 1,500 ppm, byweight based on the total weight of the composition, inorganic salt inamounts from about 25 ppm to about 3,000 ppm, preferably about 25 ppm toabout 1,000 ppm, most preferably about 50 ppm to about 500 ppm, byweight based on the total weight of the composition and, optionally,sulfonic acid salt and/or phosphonic acid salt, or combinations thereof,in amounts of from 0 to about 3,000 ppm, preferably about 100 ppm toabout 3,000 ppm, most preferably about 200 ppm to about 1,500 ppm, byweight based on the total weight of the composition.

A wide variety of inorganic salts can be used. Sodium tetraborate(Na₂B₄O₇) and calcium tetraborate (CaB₄O₇), however, are preferred. Thesulfonic acid salts that are preferred in this invention are those saltsof CF₃CF₂ (CF₂CF₂) _(n)CH₂CH₂SO₃X where X is either H or NH₄ and n=1-10,predominantly n=2-4, preferably the barium salt. A useful sulfonic acidsalt for the invention is ZONYL® BAS, which is barium salt of ZONYL TBS,both available from E.I. DuPont de Nemours and Company (“DuPont”),Wilmington, Del., USA. Other sulfonic acid salts that can be used arepotassium perfluorooctane sulfonate sold under the trademark FLUORAD®FC-95, potassium perfluorobutane sulfonate (L-7038) both available from3M, St. Paul, Minn., USA and perfluoroalkyl sulphonic acid availableunder the tradename FORAFAC® 1176 from Atofina Chemicals, Philadelphia,Pa., USA.

The preferred thermoplastic resins useful for the invention are polymershaving at least about 35 weight percent fluorine and having a melt flowrate (measured in g/10 minutes) of about 1 to about 100, preferablyabout 5 to about 35. Preferred polymers include polyvinylidene fluoride;copolymers of tetrafluoroethylene (“TFE”) and perfluoro(methyl vinylether) (“PMVE”); copolymers of TFE, PMVE and perfluoro(propyl vinylether) (“PPVE”); ethylene/chlorotrifluoroethylene (“E/CTFE”); copolymersof E/CTFE and butyl acrylate and/or hexafluoroisobutylene (“HFIB”) andcopolymers of TFE and hexafluoropropylene (“HFP”) optionally containingone or more additional monomers, including perfluoro (alkyl vinyl ether)(“PAVE”).

Thermoplastic resin, TiO₂, inorganic salt, and, optionally, the sulfonicacid salt and/or phosphonic acid salt can be combined in several ways toachieve foamable compositions, or concentrates that can be diluted tofoamable compositions By way of non-limiting examples, thermoplasticresin in powder or cube form, TiO₂, inorganic salt, and sulfonic acidsalt and/or phosphonic acid salt if desired, can be dry blended indesired proportions and melt extruded to obtain a compoundedcomposition. Alternatively, individual concentrates of the TiO₂,inorganic salt, and, optionally, sulfonic acid salt and/or phosphonicacid salt in thermoplastic resins can be blended with the same or adifferent compatible thermoplastic resin and then extruded to thedesired composition. Alternatively, one or more of the nucleating agentcomponents in powder form can be combined with concentrates of one ormore of the nucleating agent components, and the thermoplastic resin.Alternatively, one component of the nucleating agent system can first beincorporated into the thermoplastic resin by extrusion, and then thesecond component of the nucleating agent system can be added.

The foamable thermoplastic resin compositions comprising the TiO₂nucleating agent systems described herein have a variety of uses,including insulating materials for conductors or conductive material,thermal insulation, cushioning, foamed pipe and tubing and moldedarticles, (e.g. injection molded articles). In particular, thenucleating agents can be combined with thermoplastic resin and processedto obtain foamed insulation material for conductors or conductivematerial, the insulation material having a wall thickness of about 0.005to about 0.300 inches, preferably about 0.025 to about 0.200 inches whenthe nucleating agent system does not comprise sulfonic acid salt and/orphosphonic acid salt, and about 0.005 inches to about 0.025 inches whensulfonic acid salt and/or phosphonic acid salt is included.

DETAILED DESCRIPTION OF THE INVENTION

The nucleating agent systems of the invention comprise TiO₂, certaininorganic salts and, optionally, sulfonic acid salts and/or phosphonicacid salts. The nucleating agents are, generally, used in conjunctionwith a thermoplastic polymer to make foamed polymer articles, such asfoamed insulation for conductors or conductive materials. The componentsof the nucleating agent systems function synergistically to provide forfoamed polymer compositions with smaller cell size and better structurecompared to nucleating agents known in the art, such as those comprisingBN. It is discovered that TiO₂ can be used in the nucleating agentsystem at a range of about 50 ppm to about 2,000 ppm to obtain foamedpolymer articles having enhanced and acceptable performancecharacteristics. Thus, the invention allows the use of TiO₂ in smalleramounts compared to BN, such as grades SHP-325 or CTF-5 available fromCarborundum Boron Nitride, Amherst, N.Y. USA, which when used alonerequires loading requirements in the range of about 5,000 ppm to about20,000 ppm, and provide foamed polymer articles with smaller cell sizethan can be achieved with nucleating systems known in the art.

TiO₂ is a white inorganic powder that is found in nature, and can beprepared by methods such as the direct combination of titanium andoxygen, treatment of titanium salts or by chemical reaction orhydrolysis. Useful forms of TiO₂ for the nucleating agent systems of theinvention are available from DuPont under the trademark TI-PURE® GradesR-900, R-902, R-960, and R-931. TiO₂ products available from IshiharaCorporation, San Francisco, Calif., USA under the trademark TIPAQUE® andTiO₂ products under the trade names KIMERIA TiO₂ and/or TRONOX availablefrom Kerr-McGee Chemical LLC, Oklahoma City, Okla. USA are also usefulfor the invention. The TiO₂ products may have a TiO₂ content of about70% to 100%, with the balance of the product being alumina, silica,organic materials, as surface coatings, additives, fillers and/orpigments. TiO₂ having inorganic surface coating such as alumina or acombination of alumina and silica are preferred for the invention.

The nucleating agent systems comprise TiO₂ as a component of anucleating agent for thermoplastic polymers in combination with certaininorganic salts and, optionally, sulfonic acid salts and/or phosphonicacid salts. The nucleating agents comprise TiO₂ in amounts from about 50ppm to about 2,000 ppm, preferably about 100 ppm to about 1,500 ppm, byweight based on the total weight of the composition, inorganic salt inamounts from about 25 ppm to about 3,000 ppm, preferably about 25 ppm toabout 1,000 ppm, most preferably about 50 ppm to about 500 ppm, byweight based on the total weight of the composition and, optionally,sulfonic acid salts and/or phosphonic acid salts, or combinationsthereof, in amounts of from 0 to about 3,000 ppm by weight based on thetotal weight of the composition. When the composition comprises sulfonicacid salts and/or phosphonic acid salts, however, the preferred amountof such salts is about 100 ppm to about 3,000 ppm, most preferably about200 ppm to about 1,500 ppm, by weight based on the total weight of thecomposition. The term “by weight based on the total weight of thecomposition” means the weight of the components of the nucleating agentsystem plus the weight of the thermoplastic polymer and any additives,filers, pigments or other matter in the composition.

Inorganic salts useful for the invention include, those selected fromthe group consisting of lithium carbonate, calcium carbonate, calciumtetraborate, strontium carbonate, sodium carbonate, sodium tetraborate,sodium sulfite, potassium tetraborate, potassium pyrosulfate, potassiumsulfate, barium nitrate, aluminum phosphate, sodium fluorosilicate andcombinations thereof. The inorganic salts that are preferred for use inthe invention are sodium tetraborate and calcium tetraborate.

The sulfonic acid salts that can be used in the invention are metalsalts or other salts, such as hydrogen, ammonium, substituted ammoniumand quaternary ammonium, of CF₃CF₂(CF₂CF₂)_(n)CH₂CH₂SO3 X where X iseither H or NH₄ and n=1-10, predominantly n=2-4. Barium salts ofsulfonic acid, such as ZONYL BAS from DuPont, are the preferred sulfonicacid salts, however, other metal salts that can be used in the inventionare zirconium, cerium, aluminum, chromium, iron, zinc, calcium,strontium, potassium, or lithium. Other sulfonic acid salts that can beused are potassium perfluorooctane sulfonate, such as FLUORAD FC-95, andpotassium perfluorobutane sulfonate (L-7038), both available from 3 M,and FORAFAC 1176 available from Atofina Chemicals.

Phosphonic acid salts can be used in the TiO₂ nucleating agent systems,in addition to or in place of the sulfonic acid salts. Barium salts ofphosphonic acid, such as barium salt of Ashahi Guard AG-530N availablefrom Asahi Glass Co., Ltd., Tokyo, Japan are the preferred phosphonicacid salts used in the nucleating agent system, however, phosphonic acidsalts of zirconium, cerium, aluminum, chromium, iron, zinc, calcium,strontium, potassium or lithium can also be used.

Commercially available barium acid salts, such as ZONYL BAS availablefrom DuPont, can be used in the TiO₂ nucleating agent systems. Thebarium acid salts of sulfonic acid can also be made using raw materialanhydrous BaCl₂ and water borne sulfonic acid, such as ZONYL TBSavailable from DuPont, and FORAFAC® 1033 D and 1187 available fromAtofina Chemicals, according to the following bench formula and processwhich, as will be understood by one skilled in the art, can be scaled-upto make larger batch sizes. For clarity, the process is described withrespect to ZONYL TBS, however, other raw materials can be readily usedin the process. A reaction mass is formed using ZONYL TBS and anhydrousBaCl₂. The ZONYL TBS solution having approximately 30% solids is heatedto approximately 70° C. to achieve a uniform mixture. Then, 50.0 gramsof the uniform mixture of ZONYL TBS is diluted with 200 grams ofde-ionized water in 1 liter flask. After the dilution, 6.63 grams ofanhydrous BaCl₂ dissolved in approximately 20 grams of de-ionized wateris added under constant agitation. After 30 minutes, the reaction massis poured onto a coarse filter paper (P-8) attached to a Boechner funnelto obtain solids which are then washed in a screw top jar by adding 400milliliters of de-ionized water and shaking vigorously. The solids arethen recovered on filter paper and the washing procedure is repeatedtwice more. The solids are then dried for 3 days at 250° C. and thenpassed through a 500 micron Sieve. Subsequently, the dried solids areground in a 4 inch horizontal air mill to obtain the barium salt ofsulfonic acid of the desired size.

The thermoplastic resins useful in the invention include all such resinsthat can be foamed by a gas injection process and/or by use of achemical blowing agent. Preferred resins, include fluoropolymers andpolyolefins. Foamed fluoropolymers are generally foamed by continuousgas injection using nitrogen, carbon dioxide, argon, helium or other gasor gas mixture which is either soluble in or disperses in the moltenresin, using an extruder screw designed for foam operations, and anucleating agent. Nitrogen is especially suited for foaming thefluoropolymers with the TiO₂ nucleating agents because the nitrogeneasily dissolves and/or disperses in molten fluoropolymer resins, andits thermal stability is adequate to survive the holdup time in theextruder when mixed with molten resin at temperatures up to about 450°C.

A nucleating agent is needed to achieve uniform small-diametercellstructure. According to the invention, the nucleating agent systemfor fluoropolymer resins comprising TiO₂, inorganic salts and,optionally, sulfonic acid salts, and/or phosphonic acid salts provide afoamed polymer having smaller cell size than foamed polymers comprisingnucleating agents having BN, including systems where BN is combined withinorganic salts and/or sulfonic acid salts.

The gas dissolves and/or is dispersed in the molten thermoplastic resininside the extruder. By adjusting the gas pressure in the feed, theamount of gas dissolved or dispersed in the melt can be controlled. Asan alternative process, a chemical blowing agent (a chemical which isunstable at the polymer processing temperature and liberates a gas, suchas nitrogen, carbon dioxide or hydrogen) can be incorporated into thethermoplastic to provide the gas which causes bubble formation.

Foam cell formation actually starts shortly after the molten resincontaining the blowing agent passes out of the extrusion die. The gasdissolved or dispersed in the molten resin expands because of the suddendrop in melt pressure as the extrudate exits the extrusion die. Thepolymer is solidified when the extrudate is cooled, such as beingtreated with water thereby stopping foam cell growth.

The polymer resins useful in the present invention are organic polymericcompounds, preferably fluoropolymers comprising at least about 35 weightpercent fluorine, having a melt flow rate of about 1 g/10 min. to about100 g/10 min. preferably about 5 g/10 min. to about 35 g/10 min. asmeasured according to ASTM D1238 at the temperature appropriate to eachresin.

Polymers useful for the invention include:

1. Homopolymers of chlorotrifluoroethylene (CTFE), 2,2-difluoroethylene,or vinylidene fluoride.

2. Copolymers of TFE and a monomer selected from the group consisting ofCTFE, 2,2-difluoroethylene or vinylidene fluoride.

3. Copolymers of at least one monomer selected from the group consistingof TFE, CTFE, 2,2-difluoroethylene, vinylidene fluoride, andcombinations thereof, and one or more monomers selected from the groupconsisting of hexafluoropropylene (“HFP”), ethylene (“E”), terminallyunsaturated perfluoroolefins having 3-8 carbon atoms, PAVE having 3-8carbon atoms, perfluoro(alkoxy alkyl vinyl ether) having 5-12 carbonatoms, perfluoroalkyl ethylene having 3-8 carbon atoms, and monomers ofthe dioxle family, such as perfluorodioxole, perfluoro (2,2dimethyl-1,3-dioxole), and perfluoro (5 methoxy-1,3-dioxole), andcombinations thereof.

Preferred fluoropolymers include the copolymers of TFE and PMVE;copolymers of TFE, PMVE and PPVE; copolymers of TFE and PPVE; copolymersof TFE and perfluoro(ethyl vinyl ether); E/CTFE; copolymers of E/CTFEand butyl acrylate and/or HFIB; polyvinylidene fluoride and copolymersof TFE and HFP optionally containing one or more additional monomers,including PAVE.

Thermoplastic resin, inorganic salt, and, optionally, the sulfonic acidsalt and/or phosphonic acid salt can be combined in several ways toachieve foamable compositions, or concentrates that can be diluted tofoamable compositions. By way of example and not limitation,thermoplastic resin in powder or cube form, TiO₂, inorganic salt, andsulfonic acid salt and/or phosphonic acid salt, if desired, can be dryblended in desired proportions and melt extruded to obtain a compoundedcomposition. Alternatively, individual concentrates of the same or TiO₂,inorganic salt, or optionally, sulfonic acid salt and/or phosphonic acidsalt in thermoplastic resins can be blended with compatiblethermoplastic resin and then extruded to the desired composition if thedifferent thermoplastic resins are compatible. Alternatively, one ormore of the nucleating agent components in powder form can be combinedwith concentrates of one or more of the nucleating agent components, andthe thermoplastic resin. Alternatively, one component of the nucleatingagent system can first be incorporated into the thermoplastic resin byextrusion, and then the second component of the nucleating agent systemcan be added. Other processing methods know in the art may also beutilized to advantage.

The concentrations of TiO₂, inorganic salt, and, optionally, sulfonicacid salt and/or phosphonic acid salt to produce an effective foam cellnucleation system, that is a system that provides for a uniform, smallspherical cell structure without any or a significant number ofelongated cells, will depend on the foam structure being produced, thespecific resin being used, and the combination of nucleating agentcomponents. For the foamable thermoplastic resin compositions discussedherein, TiO₂ has concentration in the range of about 50 ppm to about2,000 ppm by weight, based on the total weight of the composition,preferably about 100 ppm to about 1,500 ppm. The concentration ofinorganic salt is in the range of about 25 ppm to about 1,000 ppm byweight, based on the total weight of the composition, preferably about50 ppm to about 500 ppm. Polymers having a very high dielectric constantmay require even higher level of inorganic salt, e.g. up to about 3,000ppm based on the total weight of the composition. The concentration ofthe sulfonic acid salt and/or phosphonic acid salt is generally 0 toabout 3,000 ppm, preferably about 100 ppm to about 3,000 ppm, mostpreferably about 200 ppm to about 1,500 ppm, based on the total weightof the composition. More than one type of inorganic salt, sulfonic acidsalt and/or phosphonic acid salt can be used, with combinedconcentrations as recited above, but generally a single type in eachcategory selected for the particular foam nucleating agent system isused. Polymers of the type preferred for the invention generally requirefrom about 5,000 ppm to about 10,000 ppm BN loadings, when used alone inconventional BN nucleating agent systems, for effective nucleation.

When a foamable composition concentrate is prepared, the concentrationsof the components of the foam nucleating agent system are usually somemultiple of the concentrations recited above. It is convenient to makeconcentrates with concentration of nucleating agent 10× to 20× theconcentration desired for the foamable extrusion composition, in whichcase one part of the concentrate is blended with 9 parts to 19 part ofnatural resin, i.e., resin containing no nucleating agent, respectively.The foamable composition concentrate will comprise any of the organicpolymers or fluoropolymers discussed herein and from about 500 ppm toabout 40,000 ppm, preferably about 1,000 ppm to about 30,000 ppmtitanium dioxide; about 250 ppm to about 60,000 ppm, preferably about250 ppm to about 20,000 ppm, most preferably about 500 ppm to about10,000 ppm, of the inorganic salts discussed herein and 0 to about60,000 ppm, preferably about 1,000 ppm to about 60,000 ppm, mostpreferably about 2,000 ppm to about 30,000 ppm, sulfonic acid salts,phosphonic acid salts and combinations thereof as discussed herein allby weight, based on the total weight of the concentrate, i.e. prior todilution. While the natural resin should at least be compatible with theresin used in the concentrate, it need not have the same chemicalcomposition or be of the same grade. However, it is common to usenatural resin of the same grade used as the matrix for the concentrate.

The foamable thermoplastic resin compositions of this invention areuseful for making foam structures, especially foamed insulation forconductors or conductive material, the insulation having a wallthickness of about 0.005 inches to about 0.300 inches. The mostpreferable wall thickness for TiO₂/inorganic salt system is about 0.025inches to about 0.200 inches and the most preferable wall thickness forTiO₂/inorganic salt/sulfonic acid salt and/or phosphonic acid saltsystem is about 0.005 inches to about 0.025 inches.

The invention involves a nucleating agent system comprising TiO₂,inorganic salts, preferably sodium tetraborate or calcium tetraborate,and optionally sulfonic acid salts, preferably barium salts of sulfonicacid, and/or phosphonic acid salts, preferably barium salts ofphosphonic acid. The inventor has discovered that TiO₂, which is arelatively inexpensive material, can be used in effective amounts inconjunction with the inorganic salts and, optionally, sulfonic acidsalts and/or phosphonic acid salts at much lower concentrations than BN,to achieve foamed polymer compositions having better cell structure andperformance properties. A critical range of TiO₂ of between about 50 ppmto about 2,000 ppm, preferably about 100 ppm to about 1,500 ppm, byweight based on the total weight of the composition, used in conjunctionwith the inorganic salt and, optionally, sulfonic acid salt and/orphosphonic acid salt is identified that results in foamed polymerarticles, particularly insulation for conductors and conductivematerials, including thin walled applications. Foam cell size isimportant to the performance properties of the polymer, and the cellsshould not be elongated. The foamed products made with the TiO₂nucleating agent system have unexpectedly better cell size and structurethan foamed polymers made with other agents, including BN. TiO₂ used inconjunction with inorganic salts and, optionally, sulfonic acid saltsand/or phosphonic acid salts at the low concentrations, provides forbetter nucleating, e.g. smaller cell size, than with a correspondingamount of BN and the same salts. Also, substantial cost savings can beachieved in that 1)BN, which costs about $60 to about $90 per poundcompared to $1 to $2 per pound for TiO₂, can be completely eliminatedfrom the foaming process and 2) less amount of TiO₂ is needed than BNfor more effective nucleation.

EXAMPLES

The foamable fluoropolymer compositions used in the examples wereformulated by developing a fluoropolymer blend concentrate comprisingthe nucleating agent system, and then diluting the concentrate with anatural resin. The foamable fluoropolymer resin concentrates used in thefollowing examples were prepared by combining MFA 6012 resin powder,available from the assignee of invention, Ausimont USA, Inc., Thorofare,N.J., USA under the trademark HYFLON® MFA6012, with additives comprisingthe nucleating agent system, specifically TiO₂, an in organic salt ofeither sodium tetraborate or calcium tetraborate and, optionally, abarium salt of sulfonic acid or other sulfonic acid salts, as identifiedin Table 1. The barium salt of sulfonic acid was prepared from waterborne ZONYL TBS Fluorosurfactant and BaCl₂ using the bench processdescribed herein. Certain comparative preparations were formulated asnoted.

The components of the resin concentrate are combined in a polyethylenebag, blended well by shaking and kneaded by hand, and then extrudedthrough a Model CTSE-V extruder, available from C. W. Brabender, SouthHackensack, N.J., USA, equipped with co-rotating conical twincompounding mixing screws and a strand cutter to pelletize to formconcentrate pellets. The concentrate pellets were then blended withpellets of natural fluoropolymer resin and the pellet blend was fed to astandard commercially available wire coating extruder hopper.

The concentration of additives (e.g. the components of the nucleatingagent systems) in the examples are calculated and expressed by weightbased on the total weight of the composition, e.g. fluoropolymer andadditives. The concentration of the additives were generally 20× theconcentration in the extrusion composition as set forth in Table 3, sothe ratio of natural resin to concentrate was 19/1. The specificfluoropolymer resins and additives for foamable extrusion used in theexamples are identified in Table 1, and the concentrations by weightbased on the total weight of the composition are set forth in Table 3.Unless otherwise stated, polymer resin available from Ausimont USA, Inc.under the trademark HYFLON MFA 840 with a melt flow rate of about 18.3,was employed to let down (dilute) the concentrate to a foamableextrusion composition.

In the following examples the foaming process was a continuous gasinjection foaming process carried out using nitrogen as the blowingagent. High pressure gas injection was used with a single injectionport. A Siebe 45-mm extrusion wire line available from SIEBEEngineering, Industriegebiet Fernthal, Neustadt/Wied, GERMANY was usedin a melt draw extrusion technique. An extruder length/diameter ratio of30/1 was used and the Siebe 45-mm extrusion line was equipped with ascrew designed for foaming operations. Extruder screw design allowed forthe gas injection and had mixing sections to provide a uniform melt.Unless otherwise specified a UF-O crosshead available from Unitek NorthAmerica, Cheshire, Conn., USA was used. Details of the extrusionconditions for the examples are provided in Tables 2 and 3.

Foam extrudate was characterized as follows. Capacitance and diameter ofthe foamed wire insulation were measured and recorded using the in-linemonitoring devices equipped on the Siebe extrusion line. Void fractionwas determined by calculating the density of a 12-inch length (“I”) ofinsulation. The void fraction of the insulation was determined by thefollowing method. First the conductor was removed from the primary. Theaverage diameter of the conductor (“d₁”) and the average diameter of theinsulation (“d₂”) was then determined by taking 5 readings with a USYS10bench type micrometer with a rotating sample holder (USYS 10H-04A-A DIAFHR-1 Bench Station) available from Zumbach Electronics Corp., MountKisco, N.Y., USA. The insulation was then weighed (“w”). The voidfraction was determined using the following formula:${{Void}\quad {Fraction}} = {1 - \quad \frac{w}{\left\lbrack {\rho \quad l\quad {\pi/4}} \right\rbrack \quad\left\lbrack {d_{2}^{2} - d_{1}^{2}} \right\rbrack}}$

where ρ is the density of solid polymer and w,I, d₁, and d₂ are definedabove.

Average foam cell size was determined by taking a photograph of a thincross-section at approximately 50× magnification. The average cell sizereported was calculated by averaging the largest 15 cells. The resultsof the examples are summarized in Table 3.

TABLE 1 Fluoropolymer Resins and Nucleating Agents Code Identificationor Description MFA 6012 HYFLON MFA 6012 from Ausimont MFA 840 HYFLON MFA840 from Ausimont NaTB Sodium tetraborate CaTB Calcium tetraborate TiO₂Titanium Dioxide, TI-PURE Grade R900 from DuPont XPH-561 Barium salt ofa mixture of perfluoroalkyl ethane sulfonic acids BN Boron nitride,Grade CTF-5 from Carborundum Corp.

TABLE 2 Extruder Details and Extrusion Conditions Extruder ConditionsConditions I Die Diameter (in) 0.185 Tip Diameter (in) 0.095 Screw Speed(rpm) 22-23 Melt Cone length (in)  1 Air Gap to water quench (m)  10Temperatures (° F.) Zone 1 700 Zone 2 735 Zone 3 785 Zone 4 820 Clamp740 Adapter 740 Cross-head 770 Face-Plate 780 Wire Preheat 350

Example 1

250 ppm TiO₂ was combined with 100 ppm NaTB to form the nucleating agentsystem. The results from Example 1 are set forth in Table 3 underExample 1. Surprisingly, the addition of NaTB allows TiO₂ to be anefficient nucleating agent at very low concentrations.

Example 2

To increase the void content of the foamed polymer, the gas pressure wasincreased from 2,500 psig as used in Example 1 to 3,000 psig. 250 ppmTiO₂ was combined with 100 ppm NaTB to form the nucleating agent system,and the results are set forth in Table 3. For comparison purposes, 250ppm BN and 100 ppm NaTB were tested, and the results of this comparativetest are set forth in Table 3 as Example 2 C. The results show that theTiO₂ based nucleating agent system is more efficient and has a foam cellsize 27% smaller than the corresponding BN based nucleating agent system(Example 2 C), in which the same materials were used with the TiO₂replaced with BN.

Example 3

250 ppm TiO₂, 100 ppm NaTB, and 250 ppm XPH-561, a barium salt ofsulfonic acid, were combined to form the nucleating agent system and theresults are set forth in Table 3 under Example 3. For comparisonpurposes, a combination of 100 ppm NaTB and 250 ppm the XPH-561 bariumsalt of sulfonic acid, without TiO₂, were used as the components of anucleating agent system with the same polymer under the same conditionsto determine the effect of the addition of the TiO₂ to the system. Theresults for the comparative test are set forth in Table 3 as Example 3C.Comparison of the results of Example 3 to Example 3C demonstrate thatusing concentrations of TiO₂ at the low concentration of 250 ppm withthe inorganic salt and sulfonic acid salt produces a foam cell size 24%smaller. Thus, the combination of TiO₂, inorganic salt and sulfonic acidsalt act synergistically to provide more efficient nucleation.

Example 4

2,500 ppm TiO₂ were combined with 100 ppm NaTB for the nucleating agentsystem. As reflected in Table 3 under Example 4, this amount of TiO₂proved to be too efficient and, thus, produced cells that wereelongated. Elongated cells are undesirable, particularly in thin walledapplications, because such cells can cause the foamed insulation to havea low crush force. A good crush force is required since the individualwires are twisted together (twinning). If the crush force is too low thefoamed insulation can get damaged which will produce a poor final cable.Accordingly, TiO₂ concentrations when combined with inorganic saltshould be less than about 2,500 ppm.

Example 5

2,500 ppm TiO₂, 100 ppm NaTB, and 250 ppm the XPH-561 barium salt ofsulfonic acid were combined and the results are set forth in Table 3under Example 5. Nucleating agent systems comprising 2,500 ppm TiO₂ aretoo efficient at this high loading of TiO₂, as undesirable elongatedcells result. Accordingly, TiO₂ concentrations, even when combined withinorganic salt and sulfonic acid salts, must be less than about 2,500ppm for effective nucleation.

Example 6

250 ppm TiO₂ was used alone as the nucleating agent (i.e. no inorganicsalts or suifonic acid salts were used). The results are set forth inTable 3 under Example 6. TiO₂, alone at the low concentration of 250 ppmdoes not effectively foam the polymer as only 2.6% voids were obtained.Thus, in order for TiO₂ to work at the low concentration range of theinvention, the inorganic salts and, optionally, sulfonic acid salts arenecessary and these salts act synergistically with the TiO₂ foreffective cell nucleation to obtain foamed polymers with smaller cellsize compared to systems comprising BN.

TABLE 3 Process Variables Nucleating Agents Final Properties Line XPH-Cell N₂ Speed Conductor BN TiO₂ NaTB 561 Size Void Diameter Example(psi) (ft/min) (inches) (ppm) (ppm) (ppm) (ppm) (micron) (%) (inches)Comments 1 2500 700 0.0223 — 250 100 — 67.2 24.5 0.04057 2 3000 8000.0223 — 250 100 — 63.4 31.3 0.03982 2C 3000 700 0.0215 250 — 100 — 80.832.1 0.04134 3 3000 700 0.0215 — 250 100 250 55.3 26.5 0.04036 3C 3000700 0.0215 — — 100 250 73.3 33.5 0.04157 4 2500 700 0.0215 — 2500  100 —68.1 26.2 0.04045 Elongated Cells 5 2500 700 0.0215 — 2500  100 250 78.025.9 0.04027 Elongated Cells 6 2500 700 0.0215 — 250 — — NA 2.6 0.03690No cell structure

What is claimed is:
 1. A nucleating agent system for foamable polymercompositions comprising about 50 ppm to about 2,000 ppm titanium dioxideby weight based on the total weight of the composition, about 25 ppm toabout 3,000 ppm inorganic salt selected from the group consisting oflithium carbonate, calcium carbonate, calcium tetraborate, strontiumcarbonate, sodium carbonate, sodium tetraborate, sodium sulfite,potassium tetraborate, potassium pyrosulfate, potassium sulfate, bariumnitrate, aluminum phosphate, sodium fluorosilicate and combinationsthereof by weight based on the total weight of the composition and about100 ppm to about 3,000 ppm sulfonic acid salts, phosphonic acid salts orcombinations thereof by weight based on the total weight of thecomposition.
 2. The nucleating agent system of claim 1 comprising about100 ppm to about 1,500 ppm titanium dioxide by weight based on the totalweight of the composition.
 3. The nucleating agent system of claim 1comprising from about 25 ppm to about 1,000 ppm inorganic salt by weightbased on the total weight of the composition.
 4. The nucleating agentsystem of claim 1 wherein the titanium dioxide has an inorganic surfacecoating.
 5. The nucleating agent system of claim 1 wherein the sulfonicacid salts are metal salts of CF₃CF₂(CF₂CF₂)_(n)CH₂CH₂SO₃X where X iseither H or NH₄ and n=1-10.
 6. The nucleating agent system of claim 5wherein the metal is selected from the group consisting of barium,zirconium, cerium, aluminum, chromium, iron, zinc, calcium, strontium,potassium, and lithium.
 7. The nucleating agent system of claim 1wherein the sulfonic acid salts are hydrogen, ammonium, substitutedammonium or quaternary ammonium salts of CF₃CF₂(CF₂CF₂)_(n)CH₂CH₂SO₃Xwhere X is either H or NH₄ and n=1-10.
 8. The nucleating agent system ofclaim 1 wherein the phosphonic acid salts are salts of barium,zirconium, cerium, aluminum, chromium, iron, zinc, calcium, strontium,potassium or lithium.